Choosing wire gauge & fuses

Wiring is where a lot of otherwise-good builds go wrong. The two things that matter are heat (a wire too thin for its current gets hot and can start a fire) and voltage drop (a long thin run wastes energy and dims your lights). Fuses are the safety net that stops a fault from turning the wire itself into the weak point.

Size for current and length

First find the current a circuit will carry: watts divided by volts. A 600 W inverter on 12 V can pull around 50 A under load — a surprisingly large number that needs a properly thick cable.

Then account for length. The longer the run, the thicker the wire needs to be to keep voltage drop under about 3% for sensitive loads. Low-voltage 12 V systems are far more sensitive to this than 24 V or 48 V, because the same power means much higher current.

Rule of thumb

As a starting point on 12 V: small lighting and signal runs are fine on 1.5–2.5 mm²; sockets and pumps typically want 4–6 mm²; a DC-DC charger or solar feed often needs 6–10 mm²; and inverter and main battery cables jump to 25–70 mm² depending on the inverter size. Always check a voltage-drop table for your exact run length rather than guessing.

Fuse to protect the wire

The fuse protects the cable, not the appliance. Choose a fuse rating below the safe current-carrying capacity of the wire it protects, and as close to the source (the battery) as practical — the few centimetres between a battery terminal and its fuse are the most dangerous part of any system.

Use a Class-T or MEGA fuse on the main battery positive, and a fused distribution panel for the smaller branch circuits. Every wire that leaves the battery should pass through a fuse sized for that wire.